Prolactin (PRL) is an anterior pituitary hormone recently found to be a physiologic regulator of immune responses. In addition, PRL- like molecules are synthesized by immunologically activated murine splenocytes and may function as co-mitogens for these cells. The experiments outlined in this application are designed to identify the PRL-producing cell population(s) and to determine the nature, conditions of production and co-mitogenic actions of these PRLs. The specific aims are: 1. To identify the PRL-producing splenocyte population(s), using the reverse plaque forming cell assay and cell surface markers to identify individual PRL-secreting cell types. The results of these studies will be confirmed with purified cell populations. 2. To define the mitogenic stimuli inducing PRL production in vitro, using a variety of B-cell and T-cell mitogens and quantitative analysis of secreted PRLs by Western blotting and enzyme immunoassay (EIA). 3. To determine if PRL production is specific to GO-G1 or S- phase events, by assessing PRLs in culture supernatants from purified lymphoblasts and cells blocked in G1 phase of cell cycle. 4. To determine the effects of lymphokines and regulators of pituitary prolactin secretion on PRL production during lymphoproliferative events, using Western blot analysis and EIA. 5. To determine the effects of purified PRL proteins on lymphocyte proliferation in response to T-cell and B-cell mitogens. 6. To identify the cell population(s) responding to PRL by increased clonal expansion, using specific cell surface markers to enumerate each cell type. These studies will provide information essential to determine the importance of lymphoid cell-derived PRLs in the proliferative responses of these cells. In addition, the regulation of their induction and secretion will be partially defined and a new PRL- bioassay developed. These results should therefore provide important new insight into the role of PRLs as immunoregulatory hormones and pave the way for future studies of their in vivo functions. Finally, in view of the evidence implicating PRL in neoplasia and tumor cell growth, an understanding of PRL co- mitogenesis may provide a new approach for the study of malignant diseases in man.